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SOLVENT: WARNING -- LEPTOMYCIN B IS UNSTABLE IN DMSO. DO NOT USE ANY DMSO WITH LEPTOMYCIN B. Ethanol is a better solvent for LepB than methanol for at least three reasons. First, published information indicates that LepB is more stable in ethanol (purity unchanged after 5 months at room temperature: US patent 4,771,070) than in methanol/water 70/30 (freezer temperatures required for storage for more than a few days: Sigma newsletter LifeScience 2, April 2001, page 11). Second, ethanol has a significantly higher boiling point than methanol, so ethanol solutions are expected to be less susceptible to changes in concentration due to solvent evaporation. Third, LepB is a relatively hydrophobic compound, and may have a tendency to stick to plastic. Being a much stronger organic solvent than methanol/water, ethanol reduces the potential loss of LepB when handled in plastic.

DILUTING LepB SOLUTIONS: Dilutions of LepB for dose response studies should be made in an organic solvent, preferably ethanol, with only the final dilution being made into culture medium.

STABILITY WARNING: LepB in any quantity is unstable when dried down into a film. Thus, under no circumstances should the solvent be removed from solutions of LepB, because rapid decomposition and loss of recoverable material will result.

We note that, as of August 2010, at least three suppliers, namely Selleck Chemicals LLC, Biotrend, now sell leptomycin B in a dried-down, solvent-free form. From our experience it is highly likely that the solid form will provide far less than the amounts of leptomycin B claimed on these vendors' vial labels. The original report describing leptomycin B stated quite clearly that leptomycin B is unstable as a solid and cannot be dried down and then successfully used at its expected potency. For example, we tested a sample of dried-down leptomycin B from a different vendor by treating it with methanol, an excellent leptomycin B solvent, and then doing an HPLC assay. The result was a nice, clean-looking peak that showed high purity. HOWEVER, THIS IS VERY MISLEADING -- IT WAS A NICE CLEAN PEAK, BUT WHEN CAREFULLY QUANTITATED, IT CONTAINED LESS THAN 40% OF THE EXPECTED AMOUNT OF LEPTOMYCIN B. This indicates that drying leptomycin B down to a solid probably results in partial polymerization to a highly insoluble material that cannot be redissolved. So, we predict that use of leptomycin B in dried-solid form from any source will result in large amounts of wasted time and money.

STABILITY TESTING: To test LepB stability in ethanol, we tested LepB dilutions under two different temperature conditions. One set of samples was maintained at freezer temperatures after purification and dilution, and then sent in ice to an academic laboratory in Texas. A second set of samples was heated in an oven to 60 ºC for two hours, then shipped to the same Texas lab, in the middle of August, without ice or refrigeration, and the samples were assayed for inhibition of COS cell growth. No significant differences were found between the two temperature conditions. In another test, we refluxed a sample of our LepB in ethanol (b.p. 78.5 ºC) for an hour; at the end of this period, HPLC analysis showed that decomposition products amounted to less than 1%.

SHIPPING CONDITIONS: We ship our LepB product at ambient temperature, without ice. All the information we have indicates that LepB is stable for at least many days, if not months, under ambient conditions. Thus, the extra cost of ice shipments does not appear to be justified. However, for maximum long-term stability, LepB solutions should be stored at -20 ºC or colder.

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